You know those deadbeat friends of yours on Facebook? They could end up killing your credit score and costing you a loan. At the very least, your no-account pals could bump up your interest rate.

A chilling story in the New York Observer’ BetaBeat blog this week details the efforts of several online banks that plan to analyze your social media profiles to determine how big a credit risk you are. It’s yet more evidence that, unlike Las Vegas, what happens on Facebook doesn’t stay on Facebook – and could come back to bite you in unexpected and unpleasant ways.

How are banks going to use this information? First, they’re going to use your friends list to troll for future prospects. If you just took out a line of credit against the equity in your house, maybe your friends will too – assuming they’ve got any equity left.

But that’s only the start. Per author Adrianne Jeffries:

…in the last year or so, financial institutions have started exploring ways to use data from Facebook, Twitter and other networks to round out an individual borrower’s risk profile—although most entrepreneurs working on the problem say the technology is three to five years away from mainstream adoption…

But there’s a nightmare scenario: if banks learn how to use social media, they could gather information they aren’t allowed to ask for on a credit application—including race, marital status and receipt of public assistance—or worse, to redline segments of the social graph.

In other words: choose your online friends wisely, for they may one day determine your APR.

It gets worse. Let’s say you fall a few months behind on your payments and you’ve decided to banish the bill collecting goons to voice mail. Hong Kong-based micro-lender Lenddo – which asks for your Facebook, Twitter, Gmail, Yahoo, and Windows Live logons when you sign up -- reserves the right to rat you out to all your friends. Per Lenddo’s FAQ:

As long as you don't fall behind on any Lenddo loan installments, you have complete control over your privacy settings and your information will only be shared with your permission. IF YOU FAIL TO REPAY, Lenddo MAINTAINS THE RIGHT TO NOTIFY YOUR FRIENDS, FAMILY, AND COMMUNITY. [boldface caps in original]

Also, they’ll break your legs – but only your virtual ones. And then sell you virtual crutches to hobble around on.

The problems here are several. As Jeffries notes, banks may become privy to all kinds of information that’s really none of their business, and use that to determine your creditworthiness. There’s also the Faux Friend problem; these models appear to assume your Facebook friends are your actual friends. Some may be, others certainly are not. Why should anyone’s troubled financial past impact mine (or, vice versa), especially a stranger with whom I have shared nothing more than the occasional “Like”?

This is not what we signed up for when we all agreed to engage in this massive social media experiment. First it was employers and college recruiters who began to use our tweets and status updates against us. Then potential mates and cops. Now banks? Who will be next?

Of course, these are just online small fry you can safely avoid ever having to deal with. The big banks would never stoop to such measures. Right? Wrong. Per the Observer article, the companies involved in these schemes claim to have been approached by “the who’s who of banking,” who were keenly interested in their social media parsing algorithms.

Jeffries managed to get credit reporting agency Equifax to respond to her queries; their response was telling:

“Our corporate development professionals are very aware of the opportunities to enhance our proprietary data and partner with companies who add value to the accuracy of our reporting, which helps our customers make better decisions prior to lending.”

I translate that as “we’re looking carefully at this and are prepared to jump in with both feet when the time is right.”

Will you be legally required to give your bank access to your Facebook, Twitter, LinkedIn profiles when you need credit? Probably not. But they won’t be legally required to approve your loan, either.

Dan Tynan has been writing about technology since Mark Zuckerberg was in nappies. A prolific freelance writer whose work has appeared in more than 70 publications, he is the former editor in chief of Yahoo Tech and a longtime contributing editor for InfoWorld and PCWorld.